1. Field of the Invention
The present invention relates to a heat treatment apparatus for performing a prescribed heat treatment on objects.
2. Description of the Related Art
Various apparatuses for performing prescribed heat treatments on objects are known. They are used in methods of manufacturing semiconductor devices. Among these apparatuses are: an apparatus for forming an oxide film on a semiconductor wafer; a CVD (Chemical Vapor Deposition) apparatus for forming thin films on a semiconductor wafer; and a thermal diffusion apparatus for diffusing impurities into a semiconductor wafer, thereby to form impurity regions therein.
There are two types of CVD apparatuses. The first is known as "horizontal type," and the second type as "vertical type." Recently popular is the vertical type which has a vertical processing chamber shaped like a hollow cylinder. In operation, a wafer boat (hereinafter called "boat") containing a number of wafers is inserted into the processing chamber. The processing chamber is heated, creating a high-temperature atmosphere in it. Then, a prescribed process gas is introduced into the chamber. Each wafer is thereby subjected to CVD in the high-temperature atmosphere, and a thin film is formed on the wafer. Here arises a problem. The thin film is thicker at the circumferential portion than at the center portion, inevitably because the gas is applied to each wafer from the circumferential portion to the center portion. Namely, the film cannot have thickness uniformity.
The thickness uniformity of a film is calculated by various methods. One method is to measure thickness of selected portions of the film, and then to divide the difference in the maximum and minimum thicknesses measured by two times the average thickness of the selected portion. Another method is to measure thickness of selected portions of the film, and to use the standard deviation of the thicknesses measured of these portions. The smaller the value thus calculated, the higher the thickness uniformity.
In the process of forming a film on a semiconductor wafer it is important to increase the thickness uniformity of the film in order to manufacture a semiconductor device which has stable electric characteristics. One method of improving the thickness uniformity is to use a so-called "basket-shaped boat cover." A basket-shaped boat cover is a hollow cylindrical cover made of quartz and designed to cover a boat which contains semiconductor wafers. The boat cover has a number of holes or slits through which a process gas can flow into the boat.
When the wafers in the boat are subjected to film-forming process while the boat is being covered by the boat cover, the active component of the process gas forms a film on the boat cover. The film thus formed on the boat cover controls the moderately active component of the gas, which flows into the boat through the holes of the cover and which is ultimately applied onto the surface of each wafer contained in the boat. Thus controlled, the moderately active component of the process gas forms on each wafer a film which has a sufficiently high thickness uniformity.
The use of a boat cover is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 4-206629 and Jpn. UM Appln. KOKAI Publication No. 2-131549. To form a film having increased thickness uniformity on a semiconductor wafer, it is desirable that the gap between the boat cover and each semiconductor wafer be uniform and narrow. To this end, a part of the boat cover may be attached firmly to the boat, as is described in Jpn. UM Appln. KOKAI Publication No. 2-131549.
Despite such use of a boat cover as described above, that portion of the film on each wafer is slightly thicker or thinner at the orientation flat portion than at any other portion of the wafer. This impairs the thickness uniformity of the film, and eventually results in a low yield of semiconductor IC chips. It is therefore demanded that a film be formed on the surface of a wafer, which has the same thickness at the orientation flat portion of the wafer as at any other portion thereof.
Recent progress in microtechnology has increased the integration density of semiconductor devices. In the case of dynamic random-access memories (DRAMs), 4M-DRAMs are now manufactured in large quantities, and techniques are being developed for manufacturing DRAMs of higher integration density, i.e., 16M-DRAMs and 64M-DRAMs. Therefore it has become important how to reduce the number of dust particles which may adhere to a semiconductor wafer in the process of manufacturing high-integration IC chips.
In the sate of the art, semiconductor wafers are transferred to a boat by means of a transfer arm having a wafer holder. The wafer holder has a hole having two ends, the first end opening in the top surface and the second end connected to a vacuum pump. The hole is evacuated when the vacuum pump is driven, whereby a wafer is attracted and held onto the top surface of the wafer holder. Then, the transfer arm is actuated, with the wafer holder holding the wafer, transferring the wafer to the boat. However, this transfer of wafers to a boat increases the number of dust particles which adhere to each semiconductor wafer--in two ways. First, as the wafer is attracted to the wafer holder by virtue of a relatively large suction force, it may be damaged, generating dust. Second, as the suction force is applied on the wafer, many dust particles in the ambient atmosphere are attracted onto that surface of the wafer which faces away from the surface contacting the wafer holder. The dust particles on the wafer will float in the air and adhere again to the wafer or to the adjacent wafers, during any subsequent step of manufacturing semiconductor devices. They will ultimately decrease the yield of the semiconductor devices.
As described above, in the case where a wafer is transferred to a boat by a transfer arm and is covered by a boat cover in the process of forming a thin film on the wafer, dust particles adhere to the wafer and the wafer is thicker or thinner, though slightly, at the orientation flat portion than at any other portion of the wafer. Semiconductor devices will be manufactured from the wafer but at a low yield.